Thank you for your note, but unfortunately you have directed it to the wrong person. You found this map - File:US_Slave_Free_1789-1861.gif - but it is based on maps I made for Territorial evolution of the United States. I did not make this map, and in fact did not know it existed until now. But, to me it seems the 'mottled green' is probably just an image artifact from how they were coloring in the map, it appears to me that all of the blue/mottled green states are intended to be the same color, and it was just an iffy coloring job between frames. You can see it in Maine, how it has both colors, but is clearly not cleanly striped.
You should contact User:Kenmayer, who created this map, for more information. He hasn't edited here since June, but he is recently active on Commons, so you could also try his talk page there. Good luck with your exhibit, and I'm sorry I couldn't help more! --Golbez (talk) 02:12, 4 October 2010 (UTC)
Golbez, Thank you for your response and comments. They were very helpful. Exhibitmanager (talk) 18:34, 8 October 2010 (UTC)